This invention relates to a magnetic recording medium used in perpendicular magnetic recording.
Recently, general attention is attracted to perpendicular magnetic recording intended to assure high density recording. This perpendicular magnetic recording has been proposed in substitution for the conventional longitudinal magnetic recording in which magnetization aligns along the longitudinal direction of a magnetic tape. Said perpendicular magnetic recording is affected by forming a magnetic field acting perpendicularly to the recording medium, for example, a tape, that is, in the direction of the thickness of the tape. The known magnetic recording medium used for the abovementioned perpendicular magnetic recording includes a double layer type (FIG. 1(a)) which is constructed by forming a low coercive force layer 2 on a nonmagnetic base 1, and placing on said low coercive force layer 2 a magnetic recording layer 3 indicating anisotropy perpendicularly to said low coercive force layer 2; and a single layer type (FIG. 1(b)) which is constructed by forming a magnetic recording layer 3 directly on a nonmagnetic base 1.
The following literature reports that the former double layer type (FIG. 1(a)) is far more advantageous than the latter single layer type (FIG. 1(b)) in respect of, for example, the recording and reproduction efficiency.
(i) Shinichi IWASAKI: IEEE Trans. on Magn., Vol. Mag-16 No. 1, 1980, PP 71-76 PA0 (ii) Patent Disclosure Sho54 (1979)-51804
The above-mentioned literature discloses that a Co-Cr alloy layer indicates good properties as the recording layer 3.
Further, a summary 22aA-9 of scientific lectures given in the second session of Magnetics Society of Japan reports that if formed with a smaller thickness than 0.3 .mu.m, said single Co-Cr layer does not indicate a sufficient perpendicular orientation as viewed from, for example, the process of crystalline growth. The Co-Cr recording layer formed with a small thickness has hitherto been considered to decrease in reproduced output. Therefore, the above-listed literature (ii) states that the Co-Cr recording layer should preferably have a thickness ranging between over 0.5 .mu.m and less than 3.0 .mu.m. However, a perpendicular magnetic recording medium provided with a low coercive force layer and such specified magnetic recording layer still has the drawback that said recording medium is not fully satisfactory in respect of freedom from the so-called tape noise, or the signal to noise (S/N) ratio. Therefore, demand has been made for the improvement of said conventional recording medium.